Progressive hearing loss can generally be categorizd as conductive hearing loss, sensorineural hearing loss (SNHL), or mixed hearing loss. Conductive hearing loss occurs when hearing loss is due to problems with the ear canal, ear drum, or middle ear and its bones (the malleus, incus, and stapes). Sensorineural hearing loss (SNHL) occurs when hearing loss is due to problems of the inner ear, also known as nerve-related hearing loss. Mixed hearing loss refers to a combination of conductive and sensorineural hearing loss, where there may be damage in the outer or middle ear and in the inner ear (cochlea) or auditory nerve.
“Nonsyndromic deafness” is hearing loss that is not associated with other signs and symptoms. In contrast, “syndromic deafness” involves hearing loss that occurs with abnormalities in other parts of the body. Different types of nonsyndromic deafness are generally named according to their inheritance patterns. Nonsyndromic deafness can occur at any age. About 1 in 1,000 children in the United States is born with profound deafness, and another 2 to 3 per 1,000 children are born with partial hearing loss. More than half of these cases are caused by genetic factors. Most cases of genetic deafness (70 to 80%) are nonsyndromic and the remaining cases are caused by specific genetic syndromes.
Researchers have identified more than 70 genes that, when altered, are associated with nonsyndromic deafness; however, some of these genes have not been fully characterized. Many genes related to deafness are involved in the development and function of the inner ear. Mutations in these genes contribute to hearing loss by interfering with critical steps in processing sound. Different mutations in the same gene can be associated with different types of hearing loss, and some genes are associated with both syndromic and nonsyndromic deafness. Nonsyndromic deafness can have different patterns of inheritance. 20% to 25% of nonsyndromic deafness cases are autosomal dominant (i.e., one copy of the altered gene in each cell is sufficient to result in hearing loss).
Current treatments for hearing loss include the use of hearing aids, cochlear implants and brainstem implants. Both hearing aids and cochlear implants amplify sounds to enable deaf people to hear, to distinguish environmental sounds and warning signals, and to modulate the voice and make speech more intelligible. Brain stem implants help persons who have had both acoustic nerves destroyed (e.g., by bilateral temporal bone fractures or neurofibromatosis) have some sound perception restored by means of electrodes connected to from sound-detecting and sound-processing devices directly to the brain stem. Currently, there is a need for effective treatments to mitigate progressive autosomal dominant non-syndromic hearing loss (ADNSHL).